Propeller



March 31.1925. 1,531,967

c. MACMILLAN PROPELLER Filed July 26, 1923 Ffigli A III!!! A B advanc e Im/errl: or"; Campbe l Macmxllan;

I His a I-b-k'omegj Patented Mar. 31, 1925.

4 UNITED ,.sT res PAT 1 l '7 1,531,967 ENT OFF-ICE.

CAMPBELL MACMILLAN, or SGHENECTADY, NEW YORK, Assmnofc. 'ro GENERAL ELEG- TRIC COMPANY, A CORPORATION or N W YORK.

PBOPELLEB.

Application filed July 26, 1928. Serial No. 654,010.

To all whom it may concern:

Be it known that I, CAMPBELL MACMIL- LAN, a citizen of the United States, residing at Schenectady. in the county of Schenectady, State of New York, have invented cer tain new and useful Improvements in Propellers, of which the following is a specification.

My invention relates to. marine propellers and other devices which operate through relative movement between a fluid and the surface of a solid, and has for its object the provision of means for minimizing (the fluid friction 'incident to operation of devices of this character.

Propeller losses are of two kinds which may be designated as impulsive losses and as frictional losses. Since the force for moving a vessel is produced by giving momentum of sternward velocity to the mass of the wake stream, it is impossible to dispense with the loss of kinetic energy in the wake stream, but this loss'may be diminished by 'giving a small slip velocity to a large mass of water rather than a higher slip velocity to a smaller mass; In other words, a large propeller has a higher impulsive efficiency than one of smaller diameter.

ler the torque would also disappear under these conditions.

' losses, are not essential to the operation of The frictional losses, unlike the impulsive the propeller and if they can be reduced a higher propeller efiiciency will be realized.

These losses do not vanish at zero true slip.

Therefore, for a given propeller, a reduction of 'friction causes the greatest improvement in efficiency at small slips and light loads but for practical reasons it is desirable to select a propeller of somewhat different characteristics which is suited to take full advantage of the improved efliciency at full working load. T

The formulae for fluid friction are radically difierent from those representing solid friction. The coefiicient of solid friction is directly proportional to a tangential force per unit of normal force exerted between the two surfaces and is inde endent of the area of surface in contact. The corresponding coeflicient of fluid friction is a force per sures in propeller practice run u The kineticlosses which reduce the impulsive efficiency unit area of contact surface. and is inde-' pendent of normal force at the wetted surface. The usual values of working presto about eleven pounds per square, inch 0 area, although instances of defective operation due to cavitation have a peared with pressures as low as'five poun s per square I inch.

In order to improve the efiiciency' of a 5 given propeller strong enough to absorb a specified amount of power at a given speed,

the most obvious method is to use materials of the greatest strength and smoothest surface. The possible improvements in this important consideration and, provided additional eddies are not introduced by the change, the reduction of surface .friction should be directly proportional to the reduction of surface in contact with the water. I In order to reduce surface pose to cut shallow radial grooves on the blade surfaces leaving between the grooves ridges the tops of which constitute the working surface of the .propeller. The tops of these ridges may be tilted at responding to the slip angle or may be given a small additional angle for the purpose of deflecting the water so that it jumps the groove without striking the edge of the ridge which follows it. At sufficiently high ve- 9o loaities the water will bridge the grooves tangentially without filling them and any broken water which does thrown leaving pressure and air.

My invention will be better understood on reference to the following description taken in connection with the accompanying out radially by centrifugal force drawings, and its scope will be'pointed out 100' in the appended claims.

eferring now to the drawing: Fig. 1 shows a' propeller constructed in accordance with my invention; Fig.2 is a section taken on the line AB of Fig. 1; and Fig. a is 105 a sectional view showing the arrangement of the ridges as related to the general contour of the blade surface.

Fig. 1 shows a propeller provided with blades I mounted on a hub or boss 2 and no projected friction, I p'roan angle corget into them is them filled with water vapor of low 6 varies 10 be confined to the outer portions of the blades as indicated in Fig. Assuming a 16 foot wheel, there exists a minimum centrifugal force of about thirteen and one-half I pounds per pound'of water atthe inner ends the grooves which increases to fifty-four pounds per pound at the blade tips. Water is also removed from the grooves by a form of injector action. That is to say the water which is stationary relatively to the blade 20 is in contact with stationary water opposite the blade and the relative velocity between the two bodies of water is several thousand feetper minute. Under these conditions the water in the grooves is dragged out over the sloping surfaces of the intervening ridges.

' Fig. 2 indicates the direction of propeller rotation as related to the movement of the vessel under normal operating conditions 80 and also shows the relation existing between the grooves 3'and ridges 4 which form the surface of the blade 1.

Fig. 3 shows the relation of the ridged surfaces to the average blade surface. The active surfaces of he ridges are approximately helical in outline and disposed at an angle with respect to the averagecontour of the blade surface which ma vary from 10 to 50,degrees, when half 0 removed by grooving, as-indicated by the an le 5. There is thus produced a strong 'an rugged propeller having a comparatively small working area and a correspondingly low friction loss.

My invention is not limited to the speclfic embodiment shown and described. The advantages gained from grooving a surface to iinize the friction produced through relative movement between it and a fluid are by no means confined to marine or other propellersbut may be realized in. connection with other apparatus which depends for its operation on contact between a fluid and the surface of-a solid. While the surface is I have described I and illustrated but one embodiment of my invention, it will be apparent to those skilled in the art that various changes and modifications, may be made in the apparatus without departing from the spirit and scope of my invention as set forth by the appended claims. b l/Vhat I claim as new and desire to secure l. A device having a surface arranged to move relatively to a fluid in contact therewith, .charar-terized by the fact that said surface comprises grooves disposed transversely to the direction of said relative movement and separated by ridges the top surfaces of Which-constitute the working sur-- facesof said device whereby the friction between saidfluid and said surface is reduced. 2. A device having a surface arranged to move relatively to a fluid in contact therewith, characterized by the fact that said surface comprises grooves disposed transversely tome direction of said rel'ative movement and separated byridges,'the top surfaces of saidridges being disposed at an angle with respect to the average contour of said surface, whereby friction' between said fluid and said surface is reduced.

3. A propeller comprising blades arranged to move relatively to a fluid in contact therewith and having a substantial part of their working surfaces formed of grooves disposed transversely to the direction of said movement, said grooves being separated by'ridges the top surfaces of which form the principal working surfaces of said blades, whereby I friction between said blades and said fluid is reduced.

4. A propeller ranged to move relatively to a fluid in contact therewith andhaving a substantial part of its working surfaces formed of grooves disposed transversely to the direction of said movement, said grooves being separated by ridges the top surfaces of which principal working surfaces of said blades and are disposed at an angle with respect to friction between said fluid and said blade is I my hand this 25th day of July, 1923."

CAMPBELL MACMILLAN.

Letters Patent of the United States, is

comprising a blade ar form the the average contour of said blade whereby 

